Method for making epitaxial structure

ABSTRACT

A method for making epitaxial structure is provided. The method includes providing a substrate having an epitaxial growth surface, patterning the epitaxial growth surface; placing a graphene layer on the patterned epitaxial growth surface, and epitaxially growing an epitaxial layer on the epitaxial growth surface. The graphene layer includes a number of apertures to expose a part of the patterned epitaxial growth surface. The epitaxial layer is grown from the exposed part of the patterned epitaxial growth surface and through the aperture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application: Application No. 201210122535.7, filed on Apr.25, 2012 in the China Intellectual Property Office, disclosure of whichare incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to an epitaxial structure, a method formaking the same and the application thereof.

2. Description of Related Art

Light emitting devices such as light emitting diodes (LEDs) applied onsubstrates made of group III-V nitride semiconductors such as galliumnitride (GaN) have been put into practice.

Since wide GaN substrate cannot be produced, the LEDs have been producedupon a heteroepitaxial substrate such as sapphire. The use of sapphiresubstrate is problematic due to lattice mismatch and thermal expansionmismatch between GaN and the sapphire substrate. One consequence ofthermal expansion mismatch is bowing of the GaN/sapphire substratestructure, which results in cracking and difficulty in fabricatingdevices with small feature sizes. A method for solving the problem isforming a plurality of grooves on surface of the sapphire substrate bylithography or etching. However, the process of lithography and etchingare complex, high in cost, and may pollute the sapphire substrate.

What is needed, therefore, is to provide an epitaxial method for solvingthe problem discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment of a method for making anepitaxial structure.

FIG. 2 is a flowchart of one embodiment of a method for making apatterned substrate.

FIG. 3 is a schematic view of the patterned substrate in the method ofFIG. 2.

FIG. 4 is a schematic view of one embodiment of a graphene layer havinga plurality of hole shaped apertures.

FIG. 5 is a schematic view of one embodiment of a graphene layer havinga plurality of rectangular shaped apertures.

FIG. 6 is a schematic view of one embodiment of a graphene layer havinga plurality of apertures in different shapes.

FIG. 7 is a schematic view of one embodiment of a plurality of subgraphene layers spaced from each other.

FIG. 8 is a scanning electron microscope (SEM) image of a drawn carbonnanotube film.

FIG. 9 is a schematic structural view of a carbon nanotube segment ofthe drawn carbon nanotube film of FIG. 8.

FIG. 10 is an SEM image of cross-stacked drawn carbon nanotube films.

FIG. 11 is another process of growing an epitaxial layer.

FIG. 12 is a schematic view of one embodiment of an epitaxial structurefabricated using the method of FIG. 1.

FIG. 13 is a schematic, cross-sectional view, along a line XIII-XIII ofFIG. 12.

FIG. 14 is a schematic view of another embodiment of an epitaxialstructure.

FIG. 15 is an exploded view of the epitaxial structure of FIG. 14.

FIG. 16 is a schematic view of another embodiment of an epitaxialstructure.

FIG. 17 is a flowchart of yet another embodiment of a method for makingan epitaxial structure.

FIG. 18 is a schematic view of one embodiment of the epitaxial structureof FIG. 17.

FIG. 19 is a schematic view of one embodiment of the epitaxialstructure.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present epitaxial structures and methods formaking the same.

Referring to FIG. 1, a method for making an epitaxial structure 10 ofone embodiment includes following steps:

(S11) providing a substrate 100 having an epitaxial growth surface 101;

(S12) etching the epitaxial growth surface 101 to form a patternedepitaxial growth surface; and

(S13) placing a graphene layer 110 on the patterned epitaxial growthsurface; and

(S14) epitaxially growing an epitaxial layer 120 on the patternedepitaxial growth surface.

In step (S11), the epitaxial growth surface 101 is used to grow anepitaxial layer 120. The epitaxial growth surface 101 is a clean andsmooth surface. Oxygen and carbon are removed from the surface. Thesubstrate 100 can be a single layer structure or a multiple layerstructure. If the substrate 100 is a single layer structure, thesubstrate 100 can be a single-crystal structure. The single-crystalstructure includes a crystal face which is used as the epitaxial growthsurface 101. The material of the substrate 100 can be SOI (Silicon oninsulator), LiGaO₂, LiAlO₂, Al₂O₃, Si, GaAs, GaN, GaSb, InN, InP, InAs,InSb, AlP, AlAs, AlSb, AlN, GaP, SiC, SiGe, GaMnAs, GaAlAs, GaInAs,GaAlN, GaInN, AlInN, GaAsP, InGaN, AlGaInN, AlGaInP, GaP:Zn or GaP:N.The material of the substrate 100 is not limited, as long as thesubstrate 100 has an epitaxial growth surface 101 on which the epitaxiallayer 120 can grow. If the substrate 100 is a multiple layer structure,the substrate 100 should include at least one layer of thesingle-crystal structure mentioned previously. The material of thesubstrate 100 can be selected according to the epitaxial layer 120. Inone embodiment, the lattice constant and thermal expansion coefficientof the substrate 100 is similar to the epitaxial layer 120 thereof inorder to improve the quality of the epitaxial layer 120. In anotherembodiment, the material of the substrate 100 is sapphire. The thicknessand the shape of the substrate 100 are arbitrary and can be selectedaccording to need.

In step (S12), the epitaxial growth surface 101 can be etched via amethod of wet etching or dry etching. Also referring to FIG. 2, theepitaxial growth surface 101 is etched via the wet etching method in oneembodiment. The method includes following steps:

(S121) placing a patterned mask layer 102 on the epitaxial growthsurface 101;

(S122) etching the epitaxial growth surface 101 to form a patternedepitaxial growth surface; and

(S123) removing the mask layer 102.

In step (S121), the material of the mask layer 102 can be selectedaccording to need, such as silicon dioxide, silicon nitride, siliconoxynitride, or titanium dioxide. The mask layer 102 can protect one partof the substrate 100 which is sheltered by the mask layer 102 from beingcorrupted by the solution. In one embodiment, the mask layer 102 isformed by following steps:

(S1211) depositing a silicon dioxide film on the epitaxial growthsurface 101; and

(S1212) etching the silicon dioxide film via the lithography method toform a patterned mask layer 102.

In step (S1211), the silicon dioxide film can be deposited via the CVDmethod. The thickness of the silicon dioxide ranges from about 0.3micrometer to about 2 micrometer.

In step (S1212), the silicon dioxide film is etched by following steps:

first, placing a photo resist on the silicon dioxide film;

second, exposing and developing the photo resist to form a patternedphoto resist; and

third, etching the silicon dioxide film with a solution composed of HF₄and the NH₄F to form the patterned mask layer 102.

The pattern of the mask layer 102 is arbitrary and can be selectedaccording to need. The pattern can be an array which is composed of aplurality of units. The shape of the unit can be round, rectangle,hexagonal, diamond, triangle or irregular shape or any combinations ofthem. In one embodiment, the shape of the unit is rectangle. Therectangles are parallel with each other and spaced in a certaininterval. The distance between the two adjacent units ranges from about1 μm to about 20 μm. The width of the rectangle ranges from about 1 μmto about 50 μm.

In step (S122), the epitaxial growth surface 101 is etched by a solutioncomprising sulfuric acid and phosphoric acid. A first part of theepitaxial growth surface 101 which is sheltered by the mask layer 102will be retained, and a second part of the epitaxial growth surface 101which is exposed from the mask layer 102 will be dissolved in thesolution. Thus the epitaxial growth surface 101 is patterned. The volumeratio between the sulfuric acid and the phosphoric acid ranges fromabout 1:3 to about 3:1. The etching temperature ranges from about 300°C. to about 500° C., and the etching time ranges from about 30 secondsto about 30 minutes. The etching time can be selected according to theetching depth.

Also referring to FIG. 3, the pattern of the patterned substrate 100 issimilar to that of the mask layer 102. In one embodiment, the substrate100 defines a plurality of grooves 103 parallel with each other on theepitaxial growth surface 101. The grooves 103 are depressed from theepitaxial growth surface 101 into the substrate 100. The grooves 103extend along the same direction. On the direction perpendicular with theextending direction of the grooves 103, the grooves 103 are spaced fromeach other with an interval. The interval between adjacent two of thegrooves 103 can be the same. The width of the grooves 103 ranges fromabout 1 μm to about 50 μm, and the interval between adjacent two of thegrooves 103 ranges from about 1 μm to about 20 μm. The depth of thegrooves 103 ranges from about 0.1 μm to about 1 μm. The depth of thegrooves 103 can be selected according to need. Furthermore, the depth ofthe grooves 103 can be the same.

In step (S 123), the mask layer 102 can be removed by dissolving in aHF₄ solution. Furthermore, the surface of the substrate 100 can bewashed with de-ionized water to remove the residual impurity such asHF₄.

In step (S13), the graphene layer 110 is directly in contact with thesubstrate 100, and covers the patterned epitaxial growth surface. Thegraphene layer 110 includes a first part and a second part. The firstpart covers the epitaxial growth surface 101 which located betweenadjacent two of the grooves 103. The second portion is suspended abovethe grooves 103, and not contacts with the epitaxial growth surface 101.

The graphene layer 110 can include at least one graphene film. Thegraphene film, namely a single-layer graphene, is a single layer ofcontinuous carbon atoms. The single-layer graphene is a nanometer-thicktwo-dimensional analog of fullerenes and carbon nanotubes. When thegraphene layer 110 includes the at least one graphene film, a pluralityof graphene films can be stacked on each other or arranged coplanar sideby side. The graphene film can be patterned by cutting or etching. Thethickness of the graphene layer 110 can be in a range from about 1nanometer to about 100 micrometers. For example, the thickness of thegraphene layer 110 can be 1 nanometer, 10 nanometers, 200 nanometers, 1micrometer, or 10 micrometers. The single-layer graphene can have athickness of a single carbon atom. In one embodiment, the graphene layer110 is a pure graphene structure consisting of graphene.

The single-layer graphene has very unique properties. The single-layergraphene is almost completely transparent. The single-layer grapheneabsorbs only about 2.3% of visible light and allows most of the infraredlight to pass through. The thickness of the single-layer graphene isonly about 0.34 nanometers. A theoretical specific surface area of thesingle-layer grapheme is 2630 m²·g⁻¹. The tensile strength of thesingle-layer graphene is 125 GPa, and the Young's modulus of thesingle-layer graphene can be as high as 1.0 TPa. The thermalconductivity of the single-layer graphene is measured at 5300 W·m⁻¹·K⁻¹.A theoretical carrier mobility of the single-layer graphene is 2×10⁵cm²·V⁻¹·s⁻¹. A resistivity of the single-layer graphene is 1×10⁻⁶ Ω·cmwhich is about ⅔ of a resistivity of copper. Phenomenon of quantum Halleffects and scattering-free transmissions can be observed on thesingle-layer grapheme at room temperature.

In one embodiment, the graphene layer 110 is a patterned structure. Asshown in FIGS. 4-6, the term “patterned structure” means the graphenelayer 110 is a continuous structure and defines a plurality of apertures112. When the graphene layer 110 is located on the epitaxial growthsurface 101, part of the epitaxial growth surface 101 is exposed fromthe apertures 112 to grow the epitaxial layer 120.

The shape of the apertures 112 is not limited and can be round, square,triangular, diamond or rectangular. The graphene layer 110 can have theapertures 112 of all the same shape or of different shapes. Theapertures 112 can be dispersed uniformly on the grapheme layer 110. Eachof the apertures 112 extends through the graphene layer 110 along thethickness direction. The apertures 112 can be a circular hole as shownin FIG. 4 or rectangular hole as shown in FIG. 5. Alternatively, theapertures 112 can be a mixture of circular hole and rectangular hole inthe patterned graphene layer 110, as shown in FIG. 6. Hereafter, thesize of the apertures 112 is the diameter of the circle or width of therectangle. The sizes of the apertures 112 can be different. The averagesize of the apertures 112 can be in a range from about 10 nanometers toabout 500 micrometers. For example, the sizes of the apertures 112 canbe about 50 nanometers, 100 nanometers, 500 nanometers, 1 micrometer, 10micrometers, 80 micrometers, or 120 micrometers. The smaller the sizesof the apertures 112, the less dislocation defects will occur during theprocess of growing the epitaxial layer 120. In one embodiment, the sizesof the apertures 112 are in a range from about 10 nanometers to about 10micrometers. A dutyfactor of the graphene layer 110 is an area ratiobetween the sheltered epitaxial growth surface 101 and the exposedepitaxial growth surface 101. The dutyfactor of the graphene layer 110can be in a range from about 1:100 to about 100:1. For example, thedutyfactor of the graphene layer 110 can be about 1:10, 1:2, 1:4, 4:1,2:1, or 10:1. In one embodiment, the dutyfactor of the graphene layer110 is in a range from about 1:4 to about 4:1.

As shown in FIG. 7, the term “patterned structure” can also be aplurality of patterned graphene layers spaced from each other. Each ofthe apertures 112 is defined between adjacent two of the patternedgraphene layers 110. When the graphene layer 110 is located on theepitaxial growth surface 101, part of the epitaxial growth surface 101is exposed from the aperture 112 to grow the epitaxial layer 120. In oneembodiment, the graphene layer 110 includes a plurality of graphenestrips placed in parallel with each other and spaced from each other asshown in FIG. 7.

The graphene layer 110 can be located on the epitaxial growth surface101 by transfer printing a preformed graphene film. The graphene filmcan be made by chemical vapor deposition, exfoliating graphite,electrostatic deposition, pyrolysis of silicon carbide, epitaxial growthon silicon carbide, or epitaxial growth on metal substrates.

In one embodiment, the graphene layer 110 of FIG. 4 can be made byfollowing steps:

(S131) providing a graphene film;

(S132) transferring the graphene film on the patterned epitaxial growthsurface of the substrate 100; and

(S133) creating patterns on the graphene film.

In step (131), the graphene film is made by chemical vapor depositionwhich includes the steps of: (a1) providing a substrate; (b1) depositinga metal catalyst layer on the substrate; (c1) annealing the metalcatalyst layer; and (d1) growing the graphene film in a carbon sourcegas.

In step (a1), the substrate can be a copper foil or a Si/SiO₂ wafer. TheSi/SiO₂ wafer can have a Si layer with a thickness in a range from about300 micrometers to about 1000 micrometers and a SiO₂ layer with athickness in a range from about 100 nanometers to about 500 nanometers.In one embodiment, the Si/SiO₂ wafer has a Si layer with a thickness ofabout 600 micrometers and a SiO₂ layer with a thickness of about 300nanometers.

In step (b1), the metal catalyst layer can be made of nickel, iron, orgold. The thickness of the metal catalyst layer can be in a range fromabout 100 nanometers to about 800 nanometers. The metal catalyst layercan be made by chemical vapor deposition, physical vapor deposition,such as magnetron sputtering or electron beam deposition. In oneembodiment, a metal nickel layer of about 500 nanometers is deposited onthe SiO₂ layer.

In step (c1), the annealing temperature can be in a range from about900° C. to about 1000° C. The annealing can be performed in a mixed gasof argon gas and hydrogen gas. The flow rate of the argon gas is about600 sccm, and the flow rate of the hydrogen gas is about 500 sccm. Theannealing time is in a range from about 10 minutes to about 20 minutes.

In step (d1), the growth temperature is in a range from about 900° C. toabout 1000° C. The carbon source gas is methane. The growth time is in arange from about 5 minutes to about 10 minutes.

In step (132), the transferring the graphene film includes the steps of:(a2) coating an organic colloid or polymer on the surface of thegraphene film as a supporter; (b2) baking the organic colloid or polymeron the graphene film; (c2) immersing the baked graphene film with theSi/SiO₂ substrate in deionized water so that the metal catalyst layerand the SiO₂ layer was separated to obtain a supporter/graphenefilm/metal catalyst layer composite; (d2) removing the metal catalystlayer from the supporter/graphene film/metal catalyst layer composite toobtain a supporter/graphene film composite; (e2) placing thesupporter/graphene film composite on the epitaxial growth surface 101;(f2) fixing the graphene film on the epitaxial growth surface 101 firmlyby heating; and (g2) removing the supporter.

In step (a2), the supporter material is poly (methyl methacrylate)(PMMA), polydimethylsiloxane, positive photoresist 9912, or photoresistAZ5206.

In step (b2), the baking temperature is in a range from about 100° C. toabout 185° C.

In step (c2), an ultrasonic treatment on the metal catalyst layer andthe SiO₂ layer can be performed after being immersed in deionized water.

In step (d2), the metal catalyst layer is removed by chemical liquidcorrosion. The chemical liquid can be nitric acid, hydrochloric acid,ferric chloride (FeCl₃), and ferric nitrate (Fe(NO₃)₃).

In step (g2), the supporter is removed by soaking the supporter inacetone and ethanol first, and then heating the supporter to about 400°C. in a protective gas.

In step (133), the method of creating patterns on the graphene film canbe photocatalytic titanium oxide cutting, ion beam etching, atomic forcemicroscope etching, or the plasma etching. In one embodiment, an anodicaluminum oxide mask is placed on the surface of the graphene film, andthen the graphene film is etched by plasma. The anodic aluminum oxidemask has a plurality of micropores arranged in an array. The part of thegraphene film corresponding to the micropores of the anodic aluminumoxide mask will be removed by the plasma etching, thereby obtaining agraphene layer 110 having a plurality of apertures.

In one embodiment, photocatalytic titanium oxide cutting is used topattern the continuous graphene coating. The method includes followingsteps:

(S133 a) making a patterned metal titanium layer;

(S133 b) heating and oxidizing the patterned metal titanium layer toobtain a patterned titanium dioxide layer;

(S133 c) contacting the patterned titanium dioxide layer with thecontinuous graphene coating;

(S133 d) irradiating the patterned titanium dioxide layer withultraviolet light; and

(S133 e) removing the patterned titanium dioxide layer.

In step (S133 a), the patterned metal titanium layer can be formed byvapor deposition through a mask or photolithography on a surface of aquartz substrate. The thickness of the quartz substrate can be in arange from about 300 micrometers to about 1000 micrometers. Thethickness of the metal titanium layer can be in a range from about 3nanometers to about 10 nanometers. In one embodiment, the quartzsubstrate has a thickness of 500 micrometers, and the metal titaniumlayer has a thickness of 4 nanometers. The patterned metal titaniumlayer is a continuous titanium layer having a plurality of spacedstrip-shaped openings.

In step (S133 b), the patterned metal titanium layer is heated underconditions of about 500° C. to about 600° C. for about 1 hour to about 2hours. The heating can be performed on a furnace.

In step (S133 d), the ultraviolet light has a wavelength of about 200nanometers to about 500 nanometers. The patterned titanium dioxide layeris irradiated by the ultraviolet light in air or oxygen atmosphere witha humidity of about 40% to about 75%. The irradiating time is about 30minutes to about 90 minutes. Because the titanium dioxide is asemiconductor material with photocatalytic property, the titaniumdioxide can produce electrons and holes under ultraviolet lightirradiation. The electrons will be captured by the Ti (IV) of thetitanium surface, and the holes will be captured by the lattice oxygen.Thus, the titanium dioxide has strong oxidation-reduction ability. Thecaptured electrons and holes are easy to oxidize and reduce the watervapor in the air to produce active substance such as O₂ and H₂O₂. Theactive substance can decompose the graphene material easily.

In step (S133 e), the patterned titanium dioxide layer can be removed byremoving the quartz substrate. After removing the patterned titaniumdioxide layer, the patterned graphene layer 110 can be obtained. Thepattern of the patterned graphene layer 110 and the pattern of thepatterned titanium dioxide layer are mutually engaged with each other.Namely, the part of the continuous graphene coating corresponding to thepatterned titanium dioxide layer will be removed.

In other embodiment, in step (S133 a), the patterned metal titaniumlayer can be formed by depositing titanium on a patterned carbonnanotube structure directly. The carbon nanotube structure can be acarbon nanotube film or a plurality of carbon nanotube wires. Theplurality of carbon nanotube wires can be crossed or weaved together toform a carbon nanotube structure. The plurality of carbon nanotube wirescan also be located in parallel and spaced from each other to form acarbon nanotube structure. Because a plurality of apertures is formed inthe carbon nanotube film or between the carbon nanotube wires, thecarbon nanotube structure can be patterned. The titanium deposited onthe patterned carbon nanotube structure can form a patterned titaniumlayer. In step (S133 b), the patterned titanium layer can be heated byapplying an electric current through the patterned carbon nanotubestructure. In step (S133 d), the part of the continuous graphene coatingcorresponding to the patterned carbon nanotube structure will be removedoff to form a plurality of apertures 112. Because the diameter of thecarbon nanotube is about 0.5 nanometers to about 50 nanometers, the sizeof the apertures 112 can be several nanometers to tens nanometers. Thesize of the apertures 112 can be controlled by selecting the diameter ofthe carbon nanotube.

The carbon nanotube structure is a free-standing structure. The term“free-standing structure” means that the carbon nanotube structure cansustain the weight of itself when it is hoisted by a portion thereofwithout any significant damage to its structural integrity. That is, thecarbon nanotube structure can be suspended by two spaced supports. Thus,the process of creating patterns on the continuous graphene coating canbe operated as follow. For example, first depositing titanium layer on aplurality of parallel carbon nanotube wires; second heating andoxidizing the titanium layer on the plurality of carbon nanotube wiresform titanium dioxide layer; third locating the plurality of carbonnanotube wires on the continuous graphene coating; fourth irradiatingthe plurality of carbon nanotube wires with the ultraviolet light; lastremoving the plurality of carbon nanotube wires to obtain a graphenelayer 110 having a plurality of rectangular shaped apertures 112.

In one embodiment, the carbon nanotube structure includes at least onedrawn carbon nanotube film. A drawn carbon nanotube film can be drawnfrom a carbon nanotube array that is able to have a film drawntherefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIGS. 8-9, each drawn carbon nanotubefilm includes a plurality of successively oriented carbon nanotubesegments 143 joined end-to-end by van der Waals attractive forcetherebetween. Each carbon nanotube segment 143 includes a plurality ofcarbon nanotubes 145 parallel to each other, and combined by van derWaals attractive force therebetween. As can be seen in FIG. 8, somevariations can occur in the drawn carbon nanotube film. The carbonnanotubes 145 in the drawn carbon nanotube film are oriented along apreferred orientation. The drawn carbon nanotube film can be treatedwith an organic solvent to increase the mechanical strength andtoughness and reduce the coefficient of friction of the drawn carbonnanotube film. A thickness of the drawn carbon nanotube film can rangefrom about 0.5 nanometers to about 100 micrometers. The drawn carbonnanotube film can be attached to the epitaxial growth surface 101directly.

The carbon nanotube structure can include at least two stacked drawncarbon nanotube films. In other embodiments, the carbon nanotubestructure can include two or more coplanar carbon nanotube films, andcan include layers of coplanar carbon nanotube films. Additionally, whenthe carbon nanotubes in the carbon nanotube film are aligned along onepreferred orientation (e.g., the drawn carbon nanotube film), an anglecan exist between the orientation of carbon nanotubes in adjacent films,whether stacked or adjacent. Adjacent carbon nanotube films can becombined by only the van der Waals attractive force therebetween. Anangle between the aligned directions of the carbon nanotubes in twoadjacent carbon nanotube films can range from about 0 degrees to about90 degrees. When the angle between the aligned directions of the carbonnanotubes in adjacent stacked drawn carbon nanotube films is larger than0 degrees, a plurality of micropores is defined by the carbon nanotubestructure. Referring to FIG. 10, the carbon nanotube structure is shownwith the aligned directions of the carbon nanotubes between adjacentstacked drawn carbon nanotube films at 90 degrees. Stacking the carbonnanotube films will also add to the structural integrity of the carbonnanotube structure.

A step of heating the drawn carbon nanotube film can be performed todecrease the thickness of the drawn carbon nanotube film. The drawncarbon nanotube film can be partially heated by a laser or microwave.The thickness of the drawn carbon nanotube film can be reduced becausesome of the carbon nanotubes will be oxidized. In one embodiment, thedrawn carbon nanotube film is irradiated by a laser device in anatmosphere comprising of oxygen therein. The power density of the laseris greater than 0.1×10⁴ watts per square meter. The drawn carbonnanotube film can be heated by fixing the drawn carbon nanotube film andmoving the laser device at a substantially uniform speed to irradiatethe drawn carbon nanotube film. When the laser irradiates the drawncarbon nanotube film, the laser is focused on the surface of the drawncarbon nanotube film to form a laser spot. The diameter of the laserspot ranges from about 1 micron to about 5 millimeters. In oneembodiment, the laser device is carbon dioxide laser device. The powerof the laser device is about 30 watts. The wavelength of the laser isabout 10.6 micrometers. The diameter of the laser spot is about 3millimeters. The velocity of the laser movement is less than 10millimeters per second. The power density of the laser is 0.053×10¹²watts per square meter.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes, and thus, the drawn carbonnanotube film will be shrunk into an untwisted carbon nanotube wire. Theuntwisted carbon nanotube wire includes a plurality of carbon nanotubessubstantially oriented along a same direction (i.e., a direction alongthe length of the untwisted carbon nanotube wire). The carbon nanotubesare substantially parallel to the axis of the untwisted carbon nanotubewire. More specifically, the untwisted carbon nanotube wire includes aplurality of successive carbon nanotube segments joined end to end byvan der Waals attractive force therebetween. Each carbon nanotubesegment includes a plurality of carbon nanotubes substantially parallelto each other, and combined by van der Waals attractive forcetherebetween. The carbon nanotube segments can vary in width, thickness,uniformity, and shape. The length of the untwisted carbon nanotube wirecan be arbitrarily set as desired. A diameter of the untwisted carbonnanotube wire ranges from about 0.5 nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. The twistedcarbon nanotube wire includes a plurality of carbon nanotubes helicallyoriented around an axial direction of the twisted carbon nanotube wire.Specifically, the twisted carbon nanotube wire includes a plurality ofsuccessive carbon nanotube segments joined end to end by van der Waalsattractive force therebetween. Each carbon nanotube segment includes aplurality of carbon nanotubes parallel to each other, and combined byvan der Waals attractive force therebetween. The length of the carbonnanotube wire can be set as desired. A diameter of the twisted carbonnanotube wire can be from about 0.5 nanometers to about 100 micrometers.Further, the twisted carbon nanotube wire can be treated with a volatileorganic solvent after being twisted to bundle the adjacent paralleledcarbon nanotubes together. The specific surface area of the twistedcarbon nanotube wire will decrease, while the density and strength ofthe twisted carbon nanotube wire will increase.

The graphene layer 110 can be used as a mask for growing the epitaxiallayer 120. The mask is the patterned graphene layer 110 sheltering apart of the epitaxial growth surface 101 and exposing another part ofthe epitaxial growth surface 101. Thus, the epitaxial layer 120 can growfrom the exposed epitaxial growth surface 101. The graphene layer 110can form a patterned mask on the epitaxial growth surface 101 becausethe patterned graphene layer 110 defines a plurality of apertures 112.Compared to lithography or etching, the method of forming a patternedgraphene layer 110 as mask is simple, low in cost, and will not pollutethe substrate 100.

In step (S14), the epitaxial layer 120 can be grown by a method such asmolecular beam epitaxy, chemical beam epitaxy, reduced pressure epitaxy,low temperature epitaxy, select epitaxy, liquid phase depositionepitaxy, metal organic vapor phase epitaxy, ultra-high vacuum chemicalvapor deposition, hydride vapor phase epitaxy, or metal organic chemicalvapor deposition (MOCVD).

The epitaxial layer 120 is a single crystal layer grown on the patternedepitaxial growth surface by epitaxy growth method. The material of theepitaxial layer 120 can be the same as or different from the material ofthe substrate 100. If the epitaxial layer 120 and the substrate 100 arethe same material, the epitaxial layer 120 is called a homogeneousepitaxial layer. If the epitaxial layer 120 and the substrate 100 havedifferent material, the epitaxial layer 120 is called a heteroepitaxialepitaxial layer. The material of the epitaxial layer 120 can besemiconductor, metal or alloy. The semiconductor can be Si, GaAs, GaN,GaSb, InN, InP, InAs, InSb, AlP, AlAs, AlSb, AlN, GaP, SiC, SiGe,GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN, GaAsP, InGaN, AlGaInN,AlGaInP, GaP:Zn, or GaP:N. The metal can be aluminum, platinum, copper,or silver. The alloy can be MnGa, CoMnGa, or Co₂MnGa. The thickness ofthe epitaxial layer 120 can be prepared according to need. The thicknessof the epitaxial layer 120 can be in a range from about 100 nanometersto about 500 micrometers. For example, the thickness of the epitaxiallayer 120 can be about 200 nanometers, 500 nanometers, 1 micrometer, 2micrometers, 5 micrometers, 10 micrometers, or 50 micrometers.

Referring to FIG. 11, step (S14) includes the following substeps:

(S141) placing the substrate 100 with the graphene layer 110 thereoninto a reaction chamber and heating the substrate 100 to 1100° C.˜1200°C., introducing the carrier gas and baking the substrate 100 for about200 seconds to about 1000 seconds;

(S142) cooling down the temperature to a range from about 500° C. to650° C. in the carrier gas atmosphere, introducing the Ga source gas andthe nitrogen source gas at the same time to grow the low-temperature GaNlayer;

(S143) stopping introducing the Ga source gas in the carrier gas andnitrogen source gas atmosphere, increasing the temperature to a rangefrom about 1100° C. to about 1200° C. and keeping for about 30 secondsto about 300 seconds;

(S154) keeping the temperature of the substrate 100 in a range fromabout 1000° C. to about 1100° C., introducing the Ga source gas againand the Si source gas to grow the epitaxial layer 120.

The growth of the epitaxial layer 120 includes following stages:

First stage, nucleating on the epitaxial growth surface 101 and growinga plurality of epitaxial crystal grains 1202 along the directionsubstantially perpendicular to the epitaxial growth surface 101;

Second stage, forming a continuous epitaxial film 1204 by making theepitaxial crystal grains 1202 grow along the direction substantiallyparallel to the epitaxial growth surface 101; and

Third stage, forming the epitaxial layer 120 by making the epitaxialfilm 1204 grows along the direction substantially perpendicular to theepitaxial growth surface 101.

In the first stage, the epitaxial crystal grains 1202 grow from theexposed part of the epitaxial growth surface 101 and through theapertures 112. The process of the epitaxial crystal grains 1202 growingalong the direction substantially perpendicular to the epitaxial growthsurface 101 is called vertical epitaxial growth. The first part of thegraphene layer 110 directly contacts and covers the epitaxial growthsurface 101, the epitaxial crystal grains 1202 grow from the apertures112 of the graphene layer 110. The second part of the graphene layer 110is suspended on the grooves 103, thus the epitaxial crystal grains 1202grow from the bottom of the grooves 103. While the crystal grains 1202grow to the plane of the second part of the graphene layer 110, thecrystal grains 1202 will grow out of the graphene layer 110 through theapertures 112.

In the second stage, the epitaxial crystal grains 1202 are joinedtogether to form an integral structure (the epitaxial film 1204) tocover the graphene layer 110. The epitaxial crystal grains 1202 can growalong the direction parallel to the epitaxial growth surface 101. Duringthe growth process, a plurality of holes will be formed in the epitaxiallayer 120 where the graphene layer 110 existed. The epitaxial crystalgrains 1202 grow and form a plurality of holes to enclose the graphenelayer 110. The inner wall of the holes can be in contact with or spacedfrom the graphene layer 110, depending on whether the material of theepitaxial film 1204 and the graphene layer 110 have mutual infiltration.Thus, the epitaxial film 1204 defines a patterned depression on thesurface adjacent to the epitaxial growth surface 101. The patterneddepression is related to the patterned graphene layer 110. If thegraphene layer 110 includes a plurality of graphene strips located inparallel with each other and spaced from each other, the patterneddepression is a plurality of parallel and spaced grooves. If thegraphene layer 110 includes a plurality of graphene strips crossed orweaved together to form a net, the patterned depression is a groovenetwork including a plurality of intersected grooves. The graphene layer110 can prevent lattice dislocation between the epitaxial crystal grains1202 and the substrate 100 from growing. The process of epitaxialcrystal grains 1202 growing along the direction substantially parallelto the epitaxial growth surface 101 is called lateral epitaxial growth.

In the third stage, the epitaxial layer 120 is obtained by growing for along duration of time. Because the graphene layer 110 can prevent thelattice dislocation between the epitaxial crystal grains 1202 and thesubstrate 100 from growing in step (302), the epitaxial layer 120 hasfewer defects therein.

Referring to FIGS. 12 and 13, an epitaxial structure 10 provided in oneembodiment includes a substrate 100, a graphene layer 110, and anepitaxial layer 120. A surface of the substrate 100 defines a pluralityof grooves 103 to form a patterned epitaxial growth surface. Thegraphene layer 110 is located on the patterned epitaxial growth surfaceand defines a plurality of apertures 112. The patterned epitaxialsurface of the substrate 100 is exposed through the plurality ofapertures 112. The epitaxial layer 120 is located on the graphene layer110 and contacts the exposed epitaxial growth surface through theapertures 112.

The graphene layer 110 includes a first part directly contacting withthe patterned epitaxial growth surface between adjacent two of thegrooves 103, and a second part suspended on the plurality of grooves103, and spaced from bottom of the grooves 103. In one embodiment, theplurality of grooves 103 extends along the same direction. The epitaxiallayer 120 covers the plurality of grooves 103 and connects to thesubstrate 100 through the plurality of apertures 112. The graphene layer110 suspended on the plurality of grooves is embedded into the epitaxiallayer 120. In one embodiment, the plurality of apertures 112 extendsalong the same direction and is parallel with the plurality of grooves103. Furthermore, the extending direction of the plurality of apertures112 can also be intersected with the extending direction of theplurality of the grooves 103.

The epitaxial layer 120 is coupled with the patterned epitaxial growthsurface of the substrate 100. The term “coupled” means that a surface ofthe epitaxial layer 120 forms a plurality of bulges engaged with theplurality of grooves 103. The epitaxial layer 120 defines a plurality ofholes, and the graphene layer 110 is embedded in the plurality of holes.The inner wall of the holes can be in contact with the graphene layer110. In one embodiment, the graphene layer 110 includes a plurality ofgraphene strips located in parallel with each other and spaced from eachother. A thickness of the epitaxial layer 120 ranges from about 0.5nanometers to about 1 millimeters, such as 100 nanometers to about 500nanometers, 200 nanometers to about 200 micrometers, or 500 nanometersto about 500 micrometers.

Referring to FIGS. 14 and 15, an epitaxial structure 20 provided in oneembodiment includes a substrate 100, a graphene layer 110, and anepitaxial layer 120. The epitaxial structure 20 is similar to theepitaxial structure 10 above except that the graphene layer 110 is agraphene film having a plurality of hole shaped apertures 112 arrangedin a array. Part of the epitaxial layer 120 extends through the holeshaped apertures 112 and in contact with the substrate 100.

Referring to FIG. 16, an epitaxial structure 30 provided in oneembodiment includes a substrate 100, a graphene layer 110, and anepitaxial layer 120. The epitaxial structure 30 is similar to theepitaxial structure 10 above except that the graphene layer 110 includesa plurality of graphene strips spaced from each other and arranged in aarray. An aperture 112 is defined between two adjacent graphene strips.Part of the epitaxial layer 120 extends through the apertures 112 and incontact with the substrate 100.

Referring to FIG. 17, one embodiment of a method for making an epitaxialstructure 40 includes following steps:

(S21) providing a substrate 100 having an epitaxial growth surface 101;

(S22) forming a patterned epitaxial growth surface by etching theepitaxial growth surface 101 to form a plurality of grooves 103; and

(S23) attaching a graphene layer 110 on the patterned epitaxial growthsurface; and

(S24) epitaxially growing an epitaxial layer 120 on the patternedepitaxial growth surface.

The method for making the epitaxial structure 40 is similar to themethod for making the epitaxial structure 10, except that the graphenelayer 110 in the method of making the epitaxial structure 40 is attachedon the patterned epitaxial growth surface.

In step (S23), the graphene layer 110 is directly attached and incontact with the patterned epitaxial growth surface. The graphene layer110 includes a first part directly attached on the patterned epitaxialgrowth surface between adjacent two of the grooves 103, and a secondpart directly attached on bottom surface and the sidewall of theplurality grooves 103.

In one embodiment, the graphene layer 110 includes a plurality ofgraphene strips spaced from each other, and each graphene strip iscomposed of graphene powders dispersed therein. The graphene powdersinclude a plurality of dispersed graphene grains. When the graphenelayer 110 includes graphene powders, the graphene powders can be formedinto a patterned structure by the process of dispersion, coating andetching. An aperture 112 is defined between two adjacent graphenestrips. In one embodiment, the graphene layer 110 of FIG. 17 can be madeby following steps:

step (231), making a graphene suspension with graphene powder dispersedtherein;

step (232), forming a continuous graphene coating on the epitaxialgrowth surface 101 of the substrate 100; and

step (233), creating patterns on the continuous graphene coating.

In step (231), the powder is dispersed in a solvent such as water,ethanol, N-methylpyrrolidone, tetrahydrofuran, or 2-nitrogendimethylacetamide. The graphene powder can be made by graphite oxidereduction, pyrolysis of sodium ethoxide, cutting carbon nanotube, carbondioxide reduction method, or sonicating graphite. The concentration ofthe suspension can be in a range from about 1 mg/ml to about 3 mg/ml.

In step (232), the suspension can be coated on the pitaxial growthsurface 101 of the substrate 100 by spinning coating. The rotating speedof spinning coating can be in a range from about 3000 r/min to about5000 r/min The time for spinning coating can be in a range from about 1minute to about 2 minutes.

In step (233), the method of creating patterns on the continuousgraphene coating can be photocatalytic titanium oxide cutting, ion beametching, atomic force microscope etching, or the plasma etching.

In one embodiment, photocatalytic titanium oxide cutting is used topattern the continuous graphene coating. The method includes followingsteps:

step (2331), making a patterned metal titanium layer;

step (2332), heating and oxidizing the patterned metal titanium layer toobtain a patterned titanium dioxide layer;

step (2333), contacting the patterned titanium dioxide layer with thecontinuous graphene coating;

step (2334), irradiating the patterned titanium dioxide layer withultraviolet light; and

step (2335), removing the patterned titanium dioxide layer.

In step (24), the graphene layer 110 is attached on the patternedepitaxial growth surface, and the epitaxial layer grains will grow onthe patterned epitaxial growth surface through the apertures 112 andfull fill into the plurality of grooves 103. The plurality of epitaxialcrystal grains grows to a continuous epitaxial film and eventually growsto an epitaxial layer 120.

Referring to FIG. 18, an epitaxial structure 40 provided in oneembodiment includes a substrate 100, a graphene layer 110, and anepitaxial layer 120. The epitaxial structure 40 is similar to theepitaxial structure 10 above except that the graphene layer 110 includesa plurality of graphene strips spaced from each other, and a part of thegraphene layer 110 is attached on the bottom surface and side of theplurality of grooves 103. The entire graphene layer 110 is directlyattached on the patterned epitaxial growth surface.

Referring to FIG. 19, an epitaxial structure 50 provided in oneembodiment includes a substrate 100, a graphene layer 110, and anepitaxial layer 120. The epitaxial structure 50 is similar to theepitaxial structure 10 described above except that the graphene layer110 includes a plurality of graphene powders dispersed on the patternedepitaxial growth surface of the substrate 100. The graphene powder canbe directly grown on the substrate 100.

The epitaxial structure and method for growing the epitaxial layer onthe epitaxial structure has following advantages. First, the substrateis a patterned structure having a plurality of grooves in micrometerscale, so the dislocation during the growth will be reduced. Second, thegraphene layer is a patterned structure, the thickness and the apertureis in nanometer scale, thus the dislocation is further reduced and thequality of the epitaxial layer is improved. Third, due to the existenceof the graphene layer, the contact surface between the epitaxial layerand the substrate will be reduced, and the stress between them isreduced, thus the substrate can be used to grow a relatively thickerepitaxial layer. Fourth, the graphene layer is a freestanding structure,thus it can be directly placed on the substrate, and the method issimple and low in cost. Fifth, the epitaxial layer grown on thesubstrate has relatively less dislocation, so it can be used to produceelectronics in higher performance.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making an epitaxial structure, themethod comprising: providing a substrate having an epitaxial growthsurface; forming a patterned epitaxial growth surface defining a patternby etching the epitaxial growth surface, wherein the pattern comprises aplurality of grooves on the patterned epitaxial growth surface; making agraphene layer on the patterned epitaxial growth surface, wherein themaking the graphene layer comprises forming a plurality of aperturesthrough the graphene layer to expose a part of the patterned epitaxialgrowth surface to form an exposed part; and epitaxially growing anepitaxial layer from the exposed part of the patterned epitaxial growthsurface and through the plurality of apertures.
 2. The method of claim1, wherein a width of the plurality of grooves ranges from about 1micrometer to about 50 micrometers, and an interval between adjacent twoof the plurality of grooves ranges from about 1 micrometer to about 20micrometers.
 3. The method of claim 1, wherein the graphene layercomprises graphene powder or a graphene film.
 4. The method of claim 1,wherein sizes of the plurality of apertures are in a range from about 10nanometers to about 500 micrometers.
 5. The method of claim 1, whereinthe step of making the graphene layer further comprises making thegraphene layer having a dutyfactor in a range from about 1:4 to about4:1, wherein the dutyfactor is an area ratio between a covered part tothe part of the epitaxial growth surface which is exposed.
 6. The methodof claim 1, wherein the epitaxial growth surface is etched along a firstdirection, and the plurality of grooves extends along the firstdirection, the plurality of apertures of graphene layer extends along asecond direction, and the first direction is perpendicular with thefirst direction.
 7. The method of claim 1, wherein the graphene layercomprises a graphene film consisting of a single layer of continuouscarbon atoms.
 8. The method of claim 7, wherein a first part of thegraphene layer is suspended over each of the plurality of grooves, and asecond part of the graphene layer is directly attached on the epitaxialgrowth surface between adjacent two of the plurality of grooves.
 9. Themethod of claim 1, wherein a first part of the graphene is directlyattached on the epitaxial growth surface between adjacent two of theplurality of grooves, and a second part of the graphene layer isattached on bottom surfaces and side walls of the plurality of grooves.10. The method of claim 1, wherein the step of making of the graphenelayer comprises: making a graphene film; transferring the graphene filmon to the patterned epitaxial growth surface of the substrate; andcreating patterns on the graphene film.
 11. The method of claim 10,wherein the step of creating patterns on the graphene film comprisesphotocatalytic titanium oxide cutting, ion beam etching, atomic forcemicroscope etching, or the plasma etching.
 12. The method of claim 11,wherein the step of creating patterns on the graphene film furthercomprises plasma etching the graphene film using an anodic aluminumoxide mask.
 13. The method of claim 1, wherein the step of making of thegraphene layer comprises: making a suspension liquid comprises graphenepowders; forming a continuous graphene coating by applying thesuspension liquid on the patterned epitaxial growth surface of thesubstrate; and creating patterns on the continuous graphene coating. 14.The method of claim 13, wherein the step of creating patterns on thecontinuous graphene coating comprises photocatalytic titanium oxidecutting, ion beam etching, atomic force microscope etching, or theplasma etching.
 15. The method of claim 14, wherein the step of creatingpatterns on the continuous graphene coating further comprisesphotocatalytic titanium oxide cutting, and the photocatalytic titaniumoxide cutting comprises following steps: making a patterned titaniumdioxide layer; contacting the patterned titanium dioxide layer with thecontinuous graphene coating; irradiating the patterned titanium dioxidelayer with ultraviolet light; and removing the patterned titaniumdioxide layer.
 16. The method of claim 15, wherein the step of makingthe patterned titanium dioxide layer comprises depositing titanium on apatterned carbon nanotube structure to obtain a patterned metal titaniumlayer and oxidizing the patterned metal titanium layer.
 17. The methodof claim 16, wherein the patterned carbon nanotube structure comprises acarbon nanotube film or a plurality of carbon nanotube wires.
 18. Themethod of claim 16, wherein the patterned metal titanium layer isoxidized by introducing an electric current into the patterned carbonnanotube structure.
 19. The method of claim 16, wherein the patternedcarbon nanotube structure is free-standing.